1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern. This latter approach is referred to as maskless lithography.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It will be appreciated that, whether or not a lithographic apparatus operates in stepping or scanning mode, it is vital that the patterned beam is directed onto the appropriate target portion of the substrate surface. In many circumstances, multi-layer structures are built up on the surface of the substrate as a result of a sequence of lithographic processing steps, and it is of course vital that the successive layers formed in the substrate are correctly aligned with each other. Thus, great care must be taken to ensure that the position of the substrate relative to the beam projection system is accurately known. In the past, with mask-based lithographic systems, this has generally been achieved by positioning the substrate in a known orientation on a substrate table, for example by engaging an edge of the substrate with a support surface on the substrate table, and then using a reference mark on the substrate to identify exactly where the substrate is relative to the substrate table. A metrology system is then used to control relative displacement between the substrate and the beam projection system, the reference mark establishing a datum position relative to which all displacements are measured. The reference mark has typically comprised a grating, and has been detected by observing its diffraction effects on an incident beam of the appropriate wavelength. These effects are thus a result of interaction between the incident beam and all of the elements of grating.
As the size of substrates increases it becomes more difficult to rely upon a single reference mark on a substrate to determine exactly where the substrate is relative to a datum position.
Establishing substrate position with sufficient accuracy is a particular problem in the manufacture of large devices, such as large flat panel displays (FPD's) relying upon large numbers of liquid crystal devices (LCD's) or other devices providing controllable pixels. In the case of FPD's, very large thin glass substrates are contemplated, e.g. about 1.85×2.2 meters with a thickness of less than about 1 mm. On each glass substrate, one or more panels can be defined, each panel corresponding to a single product, such as a computer monitor screen or a television (T.V.) screen with sizes ranging currently from 10 inches to 55 inches (measured diagonally from corner to corner). Within each panel area on a single substrate, LCD's each defining individual pixels are arrayed. To ensure high optical transmission efficiency and to avoid irregular visually apparent optical effects, no alignment gratings can be formed within any one panel. This is because the gratings would result in observable defects in the image on the eventual completed device.
One source of positional errors between the projection system and the substrate is thermal expansion. Even if an alignment mark is positioned in precisely the same location with respect to a beam projection system in two separate exposure steps, a structure formed on the surface of a substrate in the first step will not register with a subsequently formed structure in the second step if the temperature of the substrate has changed. This is a well known problem that has been addressed in the past by taking great care to maintain substrates at a predetermined datum temperature. However, this is difficult to achieve, particularly with large substrates. It is now possible to contemplate substrates of very large size, for example substrates having outside dimensions of the order of about two meters. Very small temperature variations across substrates of such size can result in expansions and contractions that are significant in the context of, for example, LCD display panels.
Therefore, what is needed is a system and method that allows for compensating for temperature effects during manufacturing of a substrate.